Chapter 25 - Arenes Flashcards
Molecular formula of benzene?
C6H6
Any compound containing a benzene ring is a ?
Aromatic compound/ arene
What shape is the benzene ring?
Hexagonal
Where are the delocalised electrons in benzene?
In the Pi bond , moving above and below the hexagonal ring,
How are the delocalised electrons shown in benzene?
Circle within hexagon
What is the C-C-H bond angle in benzene?
120
What is the shape of a benzene molecule?
Planar - trigonal planar
Benzene is an unsaturated molecule this means
It doesn’t only contain single bonds
The benzene ring is named as phenyl when it’s?
Branched
differences between the electrophilic addition and electrophilic substitution?
EA vs ES:
milder reagents vs catalyst
more reactive vs less reactive
easier to substitute the Br2 across the c=c in EA but harder to substitute in benzene
Why is electrophilic addition more reactive and ES?
- Pi e-s are localised between the C=C -> induces a dipole in EA,
- but in ES, Pi electrons are delocalised over the ring - doesn’t have a high enough electron density to induce a dipole in the Br2.
% yield equation?
actual MOLES / theoretical moles x 100
Actual moles equation?
% yield x TM / 100
Tm equation?
actual moles x 100 / % yield
benzene only reacts w bromine if a ? is present.
catalyst. Alkenes don’t need a catalyst to be able to react w bromine tho
Since a catalyst is needed for benzene to react w bromine?
benzene doesn’t decolourise B water so it’s not a normal alkene
The C-C bond in benzene?
- all bonds are of equal length - 0.140
- the bond length is between that of a double and single bond
what is hydrogenation?
the addition of hydrogen
How much more stable is benzene than if it actually contained 3 ordinary C=C or structured lik previously expected?
It’s (360 -208) = 152 kJmol-1 more stable
* 360 comes from (3 x-120) which is the energy required to substitute 3H2s
how do we know double bonds don’t exist in benzene?
the hydrogenation of benzene can be measured experimentally and has been found to be -208 NOT -360 as would be expected if double bonds were present.
benzene structure?
- 6 Pi electrons delocalised along the ring, this stabilises the structure.
- all 6 p orbitals overlap SIDEWAYS
- Pi bonding
- one ring of 6 delocalised e-s
Evidence for benzene’s structure?
- C-C bond lengths are equal in the delocalised system
- The delocalised system reacts by substitution rather than addition
- hydrogenation energies are lower than expected bc benzene is more stable/less reactive than a ring structure w 3 C=C
- a benzene ring does not contain only single bonds so it’s unsaturated
Fridel Crafts reactions are?
just ES reactions. This reaction allows an alkyl group or an acyl group to be substituted onto a benzene ring.
Acyl group?
e. g. methanoyl C= Oxygen single bond H
e. g. ethanoyl CH3C=O
e. g. propanoyl CH3CH2C=O
⭐ IT’S REALLY IMPORTANT WHICH?
atom carries the + charge as this is the one that attaches to the benzene ring
if a compound contains a benzene ring?
it belongs to the homologous series arenes
what is an electrophile ?
a species w a + charge/ partial + charge that can accept a pair of e-s